The present invention relates to dielectric materials for thin film electroluminescent devices. This is a companion to related application Ser. No. 813929, filed Dec. 27, 1985, concurrently herewith, by Joseph Lindmayer, and entitled "Stable Optically Transmissive Conductors, Including Electrodes for Electroluminescent Devices, and Methods for Making", the entire disclosure of which is hereby expressly incorporated by reference. The subject invention and the invention to which related application Ser. No. 813929 is directed are each improvements in the field of electroluminescent devices and, when employed together, result in highly-reliable and bright electroluminescent devices which operate without catastrophic breakdowns.
Electroluminescent devices have a long history, both as lamps and as displays. Earlier development had as its objective the development of a solid state lamp as a light source, typically in the form of a flat panel. More recently, electroluminescence has been employed in flat panel display systems, involving either pre-defined character shapes or individually-addressable pixels in a rectangular matrix.
The basic structure of an electroluminescent device is well known, and comprises an electroluminescent layer sandwiched between a pair of electrodes and separated from the electrodes by respective dielectric layers. Electroluminescence is the emission of light from a polycyrstaline phosphor solely due to the application of an electric field. While various electroluminescent materials are known, one generally accepted is ZnS as a host, with Mn as an activator.
For separating and electrically insulating the electroluminescent layer from the electrodes, a variety of dielectric materials have been proposed and employed. The semiconductor field, the solar cell field, the capacitor field, have all dealt with thin layers of insulators or oxides. Concerning thin layers of oxides there exists a variety of information on their characteristics, including dielectric constants, optical transmission, dielectric strength, development of pinholes, stability, and the like. A large amount of information or laboratory knowledge is available concerning different oxide interactions with supporting substrates or electrodes. However, there is a great deal which is not known. In addition, many of the oxide properties depend on the way they were formed. Some oxides are mixtures, such as SiO or SiO.sub.x, which have been used in electroluminescent device experiments. Silicon monoxide for example is known to be a mixture of Si and SiO.sub.2. Silicon monoxide is not stable when supporting large electric fields.
The electrodes differ from each other, depending upon whether it is the "rear" or the "front" viewing side of the device. A reflective metal, such as aluminum, is typically employed for the electrode on the "rear" side of the device, and a relatively thin optically transmissive layer of indium tin oxide (ITO) is typically employed for the electrode on the "front" side of the device. In lamp applications, both electrodes take the form of continuous layers, thereby subjecting the entire electroluminescent layer between the electrodes to the electric field. In a typical display application, the "front" and "rear" electrodes are suitably patterned so as to define row and column electrodes. Pixels are thus defined where the row and column electrodes overlap. Various electronic display drivers are well known which address individual pixels by energizing one row electrode and one column electrode at a time.
While seemingly simple in concept, the development of electroluminescent devices has met with many practical difficulties. Very generally, these practical difficulties arise from two factors. First, the devices are thin-film devices where even a small defect in a particular layer can cause a failure. Second, these thin-film devices are operated at relatively high voltages, typically ranging from 100 volts to 400 volts peak-to-peak. In this regard, electroluminescent devices are perhaps unique among solid state electronic devices in that the ZnS electroluminescent layer is operated beyond its dielectric breakdown voltage, and thus conducts, while the thin-film dielectric layers on either side are required to stop the conduction.
Manifestly, even a small defect can lead to catastrophic failure, and this has indeed been a problem with the prolonged application of large electric fields, during operation.